Recent years have seen the widespread use of fluorescent lamps for providing illumination in offices as well as in ordinary households. Fluorescent lamps in general use have a configuration in which a phosphor layer is formed on an inner face of a glass tube, and mercury and a rare gas are enclosed inside the glass tube. Further, at each end of the glass tube, an electrode is located and used to cause an electric discharge in the glass tube, which causes ultraviolet light to be generated from the mercury, and using this ultraviolet light, the phosphor layer generates visible light that then is emitted from the glass tube to the exterior.
Though characterized by its superior luminous efficiency and low power consumption as compared with an incandescent lamp, such a fluorescent lamp presents a problem in that, after a long period of use, sodium (Na) contained in the glass of a glass tube is diffused and forms an amalgam with mercury in the glass tube, so that the mercury is consumed, resulting in a decrease in luminous flux maintenance factor. In order to solve this problem, conventionally, a configuration has been proposed in which, for example, a protective film made up of inorganic particles is formed between a glass tube and a phosphor layer (see, for example, Patent Documents 1 and 2). Further, such a protective film also has the effect of reflecting ultraviolet light generated in a glass tube, thereby preventing the emission of the ultraviolet light to the exterior and increasing the utilization efficiency of the ultraviolet light to improve the luminous flux of a fluorescent lamp.
That is, Patent Document 1 proposes a fluorescent lamp including: a glass tube that is filled with mercury and an enclosed gas including a rare gas; a protective film that is made up primarily of alumina including boehmite type alumina and γ-alumina and is formed on an inner wall face of the glass tube; a phosphor layer that contains phosphor particles and is provided on this protective film; and a unit for maintaining an electric discharge in the enclosed gas.
Furthermore, Patent Document 2 proposes a fluorescent lamp including: a glass bulb; an electrode unit that is provided so as to be enclosed inside this bulb; an electric discharge maintaining medium that is enclosed in this bulb; a metal oxide film that is made up primarily of yttrium oxide whose primary particles are spherical or substantially spherical and have a diameter of 40 to 75 nm as a median value, and is formed as a mixture thereof with aluminum oxide; and a phosphor film that is formed so as to be laminated on this metal oxide film.
Patent Document 1: JP 2001-15017 A
Patent Document 2: JP 2003-51284 A
With a protective film provided between a glass tube and a phosphor layer as described above, it is possible to suppress the consumption of mercury in the glass tube and improve the utilization factor of ultraviolet light. This effect of the protective film increases with increasing thickness of the protective film. However, according to the conventional technique, such a protective film is set to have a thickness of about 0.1 μm or a thickness of at most about 0.2 μm. This is because, in a heating process for manufacturing a fluorescent lamp, a protective film having a thickness of more than 0.2 μm may peel off a glass tube due to a difference in expansion coefficient between the glass tube and the protective film. Particularly, when a protective film and a phosphor layer are formed in a straight glass tube, and then the glass tube is processed into the shape of a circular tube by heating, it has been the case that the protective film is likely to peel off at a bent portion of the glass tube. The peeling of a protective film may cause a phosphor layer to peel off as well, so that the luminous flux is lowered, resulting in a deterioration in the quality of a fluorescent lamp.